1. Field of the Invention
This invention relates to a drive shaft for ATVs (All Terrain Vehicle: a mounted type vehicle designed to traverse uneven terrains, also called a four-wheeled buggy).
2. Brief Description of the Prior Art
An ATV, which is a mounted type of four-wheeled or three-wheeled vehicle designed to traverse uneven terrains, is equipped with balloon tires to freely traverse such uneven terrains as wasteland and sandy place. The power transmission device for ATVs, as conceptually shown in FIG. 4, is constructed such that, for example, the power from an engine 21 is outputted from the output shafts on the front and rear sides via a speed change mechanism in the interior and is inputted to differential gears 24 and 25 on the front and rear sides via power transmission means 22 and 23, such as chains or propeller shafts. And, the engine power inputted to the differential gears 24 and 25 is reduced in speed by the mechanism of the differential gears 24 and 25 and is converted to a rotational power in a direction orthogonal thereto, whereupon it is transmitted to the front wheels 28 and rear wheels 29 through drive shafts 26 and 27. In the example shown in the same figure, constant velocity joints are used for joints A between the drive shaft 26 on the front side and the differential gear 24 and for joints B in the front wheels 28. In addition, there are cases where constant velocity joints are used for joints C between the drive shaft 27 on the rear side and the differential gear 25 and for joints D in the rear wheels 29. Further, when propeller shafts are used as the power transmission means 22 and 23, there are cases where constant velocity joints are used for joints E and F between the propeller shafts and the output shafts of the engine (speed change mechanism) 21 and joints G and H in the differential gears 24 and 25.
FIG. 5 shows the drive shaft 26 on the front side. In order to allow the drive shaft 26 to make angular displacement and axial displacement following the movement of the front wheel 28 during cornering, traversing uneven terrains or the like movement, a slide type constant velocity joint 30 and a fixed type constant velocity joint 31 are used in pair for joining the drive shaft 26. Here, the fixed type constant velocity joint means a constant velocity joint that allows only an angular displacement between two shafts, while the slide type constant velocity joint means a constant velocity joint that allows not only an angular displacement between two shafts but also an axial displacement (plunging). In the example shown in the same figure, the inboard side of the drive shaft 26 is joined to the differential gear 24 (at the joining section A) through the slide type constant velocity joint (double offset type constant velocity joint, hereinafter referred to as “DOJ”) 30, while the outboard side of the drive shaft 26 is joined to the wheel 28 (at the joining section B) through a fixed type constant velocity joint (Rzeppa type constant velocity joint: ball fixed joint, hereinafter referred to as “BJ”) 31. The numerals 32 and 33 denote boots for the constant velocity joints.
Heretofore, as the DOJ and BJ, those for passenger cars have been frequently converted to be used as such. Refer to FIGS. 6 and 7 of Japanese Patent Application Laid Open under No. 2001-97063.
Since vehicle weight restrictions are severe particularly for ATVs, further weight reduction or size compaction has been required of the drive shafts thereof. Further, since ATVs are small in size, narrow in width and high in vehicle height, the normal working angle of the constant velocity joints installed in the drive shaft is nearly twice that of those for passenger cars. For this reason, for passenger car specifications, the working stability of the constant velocity joints would be impaired depending on usage conditions or the like. Further, about half of the durability (life) of constant velocity joints for passenger cars or the like is sufficient for constant velocity joints for ATV as considered from balance between market performance and the term of guarantee; therefore, considered on the basis of passenger car specifications as they are, there is a feeling of excessive quality consciousness. As to the frequency of use, about half for passenger car specifications is sufficient as considered from balance with vehicle speed, and the same may be said. On the other hand, in the aspect of strength, such as twist strength, the same degree as for passenger car specifications is required.
In this connection, drive shaft for passenger cars employ Hytrel or other resin boots for constant velocity joint boots 33 on the outboard side where they are frequently subjected to flying sands or disturbances. However, the resin boot, which is high in rigidity, performs the function of increasing the resistance to the bending of the constant velocity joint. Particularly, concerning the drive shaft on the front side for driving the front wheels, i.e., the steering wheel which takes steer angles through steering links, the resistance to the bending of the constant velocity joint due to the hardness of the resin boots gives a large influence to the steering feeling. The influence on the steering feeling due to the hardness of such resin boots could be ignored in the case of passenger cars in which power steering is employed. However, in the case of steering mechanism for ATVs, since the handle bar is mechanically joined to the steering links, the bending resistance of the drive shaft on the front side directly influences the steering feeling.